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Letter 8113

Treat, Mary to Darwin, C. R.

20 Dec 1871

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    Describes fly-catching activity of Drosera longifolia.

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    Experiments on Papilio asterias; sex of adult determined by length of larval feeding time.


Vineland, New Jersey,

Dec. 20, 1871.

Mr. Darwin:

Dear Sir,

Experimenting with Papilio asterias, Cramer, I learned to distinguish the sex in the larva state—the female being larger than the male—and this led me to try to control the sex.

My first experiments were a a year ago last summer, some three or four hundred miles inland, where I had much better success than I had here last summer near the coast.

The larvae of my first experiment were of the first brood, so that I only had to wait a few days for their final transformation. These larvae fed on two quite dissimilar Umbelliferous plants—the Poison Hemlock (Conium maculatum), and the Caraway of the gardens (Carum Carni). I could always distinguish the larvae that fed on the Poison Hemlock from those that fed on the Caraway; but with the butterflies there was little or no marked difference in their general appearance.

I noticed that the female larva fed longer than that of the male. So taking several larvae of the same age, I found some specimens were inclined to leave their food several days earlier than others, and these always proved to be males. It then occurred to me to try to induce some of these male larvae to feed longer; so, after they had wandered from their food, and even selected places for their transformations, (of course not fixed), if I disturbed them, made them leave their places, and coaxed them with a fresh supply of their favorite food, I could almost invariably induce them to eat from ten days to two weeks longer, when all such ones would be females— If it was a larva that fed on Caraway, I tempted it with the tenderest, and freshest leaves and flowers of the same plant. It was never hungry enough at that stage to be induced to change its food,—though they will change in their earlier stages rather than starve, but evidently they do not like to change even then, and frequently fail to transform when their food is thus changed. They can be induced to change their food to the nearest allied species of plants with less difficulty—

On the other hand, when a larva had become the right size to produce a male, if I cut off its supply of food, even when it was eating greedily, it would wander about perhaps a little longer, as if in search of food, but finally it almost always changed to the chrysalis, and such a chrysalis always produced a male butterfly.

Last summer a fit of sickness prevented my experimenting with the first brood of these butterflies; and with the second brood—although I procured many specimens before the first moult—I was only able to rear about one in ten. An unnamed tiny Microgaster was their fatal enemy. But the few larvae that I succeeded with, behaved precisely as those of the previous year. Their food plant was Archangelica hirsuta. I have the chrysalids of my last experiment carefully marked, and am fully confident that I know which will produce the different sexes.

Now that I am writing I will give you my observations on Drosera, which have escaped the notice of botanists. I had two or three species of these pretty plants growing for window ornaments; and soon saw that D. longifolia was a fly-trap of considerable power. The unlucky fly—a common house fly—would no sooner be caught by the sticky glands of the leaf, than the blade would at once commence to fold about its victim; it folded from the apex to the stem of the leaf, after the manner of its vernation.

Closer and closer it held the poor fly in its embrace, until it ceased its struggles, when it soon became partly absorbed by the plant.— Prof. Gray will give my observations on this plant in his new edition of ``How Plants Grow''.

I do not know that my experiments can be of use to you, but I thought perhaps they might interest you. A life time of observation and experiments could not repay the debt of gratitude we owe you.

Yours most respectfully, | Mrs. Mary Treat.

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    This letter is not yet published in the edition of The correspondence of Charles Darwin; it will appear in volume 19, due to be published in 2011. The text is being made available here ahead of print publication as part of the `Darwin and Gender' project funded by The Bonita Trust.
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